The modulation of O-phosphotyrosyl-utilizing signal transduction processes is being addressed through the development of site-directed inhibitors of protein-tyrosine kinases (PTKs). Two approaches are being pursued: In the first approach, hydroxylated aryl-containing analogues are being prepared which are intended to inhibit the binding of tyrosyl-containing substrates within the catalytic site. Included among these are a series of bicyclic compounds which represent conformationally constrained analogues of the known PTK inhibitor, alpha-cyano- 3,4-dihydroxycinnamamide. Inhibitor testing is currently being conducted in the following PTK systems: epidermal growth factor receptor (EGF-R); p56(lck); platelet derived growth factor receptor (PDGF-R). In the second approach, phosphonomethyl phenylalanine (Pmp) as well as monofluoro- and difluoro-Pmp derivatives have been prepared and incorporated into peptides as hydrolytically stable mimetics of O- phosphotyrosyl residues. Replacement of the O-phosphotyrosyl residue (Y*) in the peptide "Y*VPML" gave "(Pmp)VPML", which was able to inhibit the src homology region 2 "SH2"-mediated association of PDGF-R with the p85 subunit of phosphoinositol-3 (PI3) kinase. Additional peptides " G (Pmp)) VPML" and "cyclo[G(Pmp)VPML] were also effective inhibitors. Efforts are currently in progress to prepare peptidomimetics based on these peptides, which have the ability to inhibit specific SH2-interactions.